At 02:28 AM 12/19/2012, Scott Daniel - Turbovans wrote:
>PCM = Powertrain Control Module ?
>
>  ( I know it's not what you mean .
>so what is it plz. )

PCM stands for Pulse Code Modulation (how your computer does audio),
see here: https://en.wikipedia.org/wiki/Pulse-code_modulation

Mike fat-fingered it; he actually meant to write PWM, Pulse Width
Modulation; see here:
https://en.wikipedia.org/wiki/Pulse-width_modulation .  In essence
it's controlling power to a load by applying full power in pulses,
varying the width of the pulses to increase or decrease average power
delivered to the load.  Wall-switch light dimmers for incandescent
lamps work this way, using a device called an SCR or silicon
controlled rectifier.  It starts conducting when given a trigger
signal, and automatically stops conducting when current stops
flowing.  So for bright light the dimmer triggers it early in the
voltage half-cycle, and for dim light late in the half-cycle.  Either
way it stops by itself at the end of the half-cycle.  The circuit
generates electrical noise when it switches, but because it's
essentially a switch rather than a variable resistance, it doesn't
heat up to speak of.  Such a circuit for the heater blower would be
slightly more complex because it would have to be actively switched
off as well as on.

You can see PWM circuits in use for LED taillights on some cars -
sweep your eyes rapidly back and forth (very easy with binoculars)
across the light, and if it separates into a trail of dots it's being
driven by a PWM controller (or AC, of course).  We used to use the
binocular trick on Scamp, to find aid-to-navigation lights mixed in
with street lights and such.  The buoys ran on DC and gave a steady
streak of light, but street lights and conventional fluorescents
blink on and off and even incandescent house lights have a wavy appearance.

Yours,
David